The present invention relates, in general, to electronics, and more particularly, to semiconductors, structures thereof, and methods of forming semiconductor devices.
In the past, the semiconductor industry utilized various methods and structures to form voltage regulator circuits that could be used to supply a regulated voltage to a load. In an application to use the regulator to supply power to a load, a filter capacitor generally was connected to the output of the voltage regulator in order to filter noise from the voltage and to provide stored energy to the load or to form a main pole in the regulator control loop. In some cases, the equivalent series resistance of the capacitor increased or the capacitor value decreased which caused an increase in the amount of noise in the output voltage or could also cause instability of the control loop. In other cases it was possible for the connections between the capacitor and the output of the voltage regulator to become damaged so that the capacitor was no longer connected thereby also causing an increase in the amount of noise on the output voltage or control loop instability since the filtering effect of the capacitor was no longer effective.
FIG. 1 schematically illustrates a prior art circuit that was utilized to detect a missing capacitor. The circuit of FIG. 1 utilized a peak detector to detect when the a.c. portion of the output voltage increased above a specific value. The regulator included a feedback circuit illustrated by resistors R1-R3. At a periodic interval, a periodic pulse was applied to transistor T1 to periodically enable transistor T1 and short the value of resistor R3 thereby periodically changing the value of the resistor divider and the feedback voltage. During the time interval when the periodic pulse was applied to transistor T1 a peak detector would check the output voltage for increased value of the a.c. portion of the output voltage. If an increase was detected, it was assumed that the capacitor was missing. However, periodically changing the value of the resistor divider also caused undesirable variations and overshoot of the output voltage. Another problem was that EMI could also induce a.c. signals on the output voltage and the circuit of FIG. 1 could not detect the difference between EMI disturbances and a missing capacitor.
Accordingly, it is desirable to have a method and circuit to detect a missing capacitor and to determine the difference between externally coupled EMI disturbances and a missing capacitor.
For simplicity and clarity of the illustration, elements in the figures are not necessarily to scale, and the same reference numbers in different figures denote the same elements. Additionally, descriptions and details of well-known steps and elements are omitted for simplicity of the description. As used herein current carrying electrode means an element of a device that carries current through the device such as a source or a drain of an MOS transistor or an emitter or a collector of a bipolar transistor or a cathode or anode of a diode, and a control electrode means an element of the device that controls current through the device such as a gate of an MOS transistor or a base of a bipolar transistor. Although the devices are explained herein as certain N-channel or P-Channel devices a person of ordinary skill in the art will appreciate that complementary devices are also possible in accordance with the present invention. It will be appreciated by those skilled in the art that the words during, while, and when as used herein relating to circuit operation are not exact terms that mean an action takes place instantly upon an initiating action but that there may be some small but reasonable delay, such as a propagation delay, between the reaction that is initiated by the initial action. Additionally, the term while means that a certain action occurs at least within some portion of a duration of the initiating action. The use of the word approximately or substantially means that a value of an element has a parameter that is expected to be very close to a stated value or position. However, as is well known in the art there are always minor variances that prevent the values or positions from being exactly as stated. It is well established in the art that variances of up to at least ten per cent (10%) (and up to twenty per cent (20%) for semiconductor doping concentrations) are reasonable variances from the ideal goal of exactly as described. When use in reference to a state of a signal, the term asserted means an active state of the signal and inactive means an inactive state of the signal. The actual voltage value or logic state (such as a “1” or a “0”) of the signal depends on whether positive or negative logic is used. Thus, asserted can be either a high voltage or a high logic or a low voltage or low logic depending on whether positive or negative logic is used and negated may be either a low voltage or low state or a high voltage or high logic depending on whether positive or negative logic is used. Herein, a positive logic convention is used, but those skilled in the art understand that a negative logic convention could also be used.